The present invention relates generally to a power transfer system for controlling the distribution of drive torque between the front and rear drivelines of a four-wheel drive vehicle. More particularly, the present invention relates to a full-time transfer case having a two-speed gear reduction unit and an interaxle differential integrated into a planetary gear assembly, a range shift mechanism for establishing high-range and low-range drive modes, and a controllable bi-directional overrunning clutch for controlling interaxle slip between the front and rear drivelines.
Four-wheel drive vehicles are in great demand due to the enhanced on and off road traction control they provide. In many four-wheel drive vehicles, a transfer case is installed in the drivetrain and is normally operable to deliver drive torque to the primary driveline for establishing a two-wheel drive mode. The transfer case is further equipped with a clutch assembly that can be selectively or automatically actuated to transfer drive torque to the secondary driveline for establishing a four-wheel drive mode. These xe2x80x9cmodexe2x80x9d clutch assemblies can range from a simple dog clutch that is operable for mechanically shifting between the two-wheel drive mode and a xe2x80x9clockedxe2x80x9d (i.e., part-time) four-wheel drive mode to a more sophisticated automatically-actuated multi-plate clutch for providing an xe2x80x9con-demandxe2x80x9d four-wheel drive mode.
On-demand four-wheel drive systems are able to provide enhanced traction and stability control and improved operator convenience since the drive torque is transferred to the secondary driveline automatically in response to lost traction at the primary driveline. An example of passively-controlled on-demand transfer case is shown in U.S. Pat. No. 5,704,863 where the amount of drive torque transferred through a pump-actuated clutch pack is regulated as a function of the interaxle speed differential. In contrast, actively-controlled on-demand transfer cases include a clutch actuator that is adaptively controlled by an electronic control unit in response to instantaneous vehicular operating characteristics detected by a plurality of vehicle sensors. U.S. Pat. Nos. 4,874,056, 5,363,938 and 5,407,024 disclose various examples of adaptive on-demand four-wheel drive systems.
As yet a further alternative, some vehicles are equipped with a full-time power transfer system having a transfer case with a center differential that functions to permit interaxle speed differentiation while transferring drive torque to both the front and rear drivelines. To minimize loss of traction due to wheel slip, most full-time transfer cases are also equipped with a clutch assembly for limiting speed differentiation and biasing the torque transferred across the center differential. Typically, the types of clutch assemblies used in full-time four-wheel drive systems to control interaxle slip across the center differential are similar to the passively and actively-controlled mode clutch assemblies used in on-demand four-wheel drive systems. Exemplary, full-time transfer cases are disclosed in commonly-owned U.S. Pat. Nos. 5,697,861 and 5,702,321.
Due to the cost and complexity associated with actively-controlled clutch assemblies, recent efforts have been directed to the use of overrunning clutches that can be easily controlled to provide various operating modes. For example, U.S. Pat. No. 5,993,592 illustrates a pawl-type controllable overrunning clutch assembly installed in a transfer case and which can be shifted between various drive modes. U.S. Pat. No. 6,092,635 discloses a hydraulically-actuated multi-function controllable overrunning clutch assembly that is noted to be operable for use in vehicular power transmission mechanisms. Likewise, U.S. Pat. Nos. 5,924,510, 5,951,428, 6,123,183, and 6,132,332 each disclose a controllable multi-mode overrunning clutch installed in a transfer case and which is actuated using an electromagnetic clutch. In view of this recent interest, a need exists to continue development of controllable bi-directional overrunning clutch assemblies which provide improved structure, robust operation, and reduced packaging for use in four-wheel drive transfer cases.
Finally, in an effort to minimize the overall size of full-time two-speed transfer cases, it has been proposed to incorporate the gear reduction unit and the interaxle differential into a common planetary gear assembly. For example, commonly-owned U.S. Pat. No. 5,902,205 discloses a full-time two-speed transfer case equipped with an integrated planetary gearset which is operable for establishing full-time high-range and low-range four-wheel drive modes through on-the-move shifting of a synchronized range shift mechanism. While such an arrangement provides a compact construction, there is a continuing need to develop alternatives which meet modern requirements for low noise and weight while advancing the state of the four-wheel drive art.
It is therefore an object of the present invention to provide a transfer case for a full-time four-wheel drive vehicle having a planetary gear assembly which integrates a two-speed gear reduction unit and an interaxle differential into a common arrangement.
As an additional object of the present invention, the full-time two-speed transfer case includes a range shift mechanism which can be selectively actuated for establishing a full-time four-wheel high-range drive mode, a neutral mode, and a full-time four-wheel low-range drive mode.
According to another object of the present invention, the full-time two-speed transfer case includes a controllable multi-mode bi-directional overrunning clutch assembly which is operably associated with the outputs of the planetary gear assembly for limiting speed differentiation and controlling the drive torque distribution therebetween in response to the occurrence of slip between the front and rear output shafts.
It is a further object of the present invention to provide a power-operated actuator to control shifting of the bi-directional overrunning clutch assembly between its distinct modes in response to mode signals received by a controller unit.
According a preferred embodiment of the present invention, the planetary gear assembly is operably installed between and input shaft and front and rear output shafts of the transfer case and is constructed in a compact arrangement. The planetary gear assembly includes a first planetary gearset and a second planetary gearset which are interconnected by a common carrier. The first planetary gearset is operably installed between the input shaft and the second planetary gearset for driving the carrier at either of a direct speed ratio (i.e., high-range) or a reduced speed ratio (i.e., low-range) relative to the input shaft. The carrier acts as the input to the second planetary gearset which has first and second outputs respectively connected to the rear and front output shafts of the transfer case. Thus, the second planetary gearset functions as an interaxle differential for permitting speed differentiation and distributing drive torque between the front and rear output shafts of the transfer case.
The present invention is also directed to integration of a controllable, multi-mode, bi-directional overrunning clutch assembly and a mode shift system into a full-time four-wheel drive transfer case for limiting slip between a primary output shaft and a secondary output shaft. The clutch assembly includes a first ring journalled on a first rotary member, a second ring fixed to a second rotary member, and a plurality of rollers disposed in opposed cam tracks formed between the first and second rings. The first ring is split to define an actuation channel having a pair of spaced end segments. An actuator ring is moveable between positions engaged with and released from the end segments of the first ring. The mode shift system is operable to control movement of the actuator ring for establishing an automatic four-wheel drive mode and a locked four-wheel drive mode.
In accordance with one particular embodiment of the bi-directional overrunning clutch assembly of the present invention, the first ring is journalled on a sprocket driven by the second output of the second planetary gearset and the second ring is driven by the rear output shaft. Thus, the invention provides for installing the controllable, multi-mode, bi-directional overrunning clutch assembly in association with the sprocket to permit significant axial length reductions for the transfer case.
Further objects, advantages and features of the present invention will become readily apparent to those skilled in the art by studying the following description of the preferred embodiment in conjunction with the appended drawings which are intended to set forth the best mode currently contemplated for carrying out the present invention.